Miniaturized Fourier-transform Raman spectrometer systems and methods

ABSTRACT

State-of-the-art portable Raman spectrometers use discrete free-space optical components that must be aligned well and that don&#39;t tolerate vibrations well. Conversely, the inventive spectrometers are made with monolithic photonic integration to fabricate some or all optical components on one or more planar substrates. Photonic integration enables dense integration of components, eliminates manual alignment and individual component assembly, and yields superior mechanical stability and resistance to shock or vibration. These features make inventive spectrometers especially suitable for use in high-performance portable or wearable sensors. They also yield significant performance advantages, including a large (e.g., 10,000-fold) increase in Raman scattering efficiency resulting from on-chip interaction of the tightly localized optical mode and the analyte and a large enhancement in spectral resolution and sensitivity resulting from the integration of an on-chip Fourier-transform spectrometer.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the priority benefit, under 35 U.S.C. § 119(e),of U.S. Application No. 62/542,608, filed Aug. 8, 2018, and entitled“Miniaturized Fourier-Transform Raman Spectrometer System,” which isincorporated herein by reference in its entirety.

BACKGROUND

A Raman spectrometer or Raman spectrophotometer is a device thatoptically probes the vibrational, rotational, and low-frequency modes ofa solid, liquid, or gaseous chemical or material. It can accuratelyquantify the chemical structure of an unknown substance. A Ramanspectrometer typically operates by first illuminating a sample with asingle frequency laser in the visible or near-infrared wavelengthregion. A fraction of the light that scatters from the sample isconverted to a higher optical frequency (anti-Stokes shifted), andanother fraction is converted to a lower optical frequency (Stokesshifted). The new frequencies of this Stokes and anti-Stokes shiftedlight (also referred to as Raman-shifted light) correspond to theintrinsic energy levels of the substance being sensed, and they can beused to uniquely identify the chemical or material, as shown in FIG. 1.

Conventional Raman spectrometer systems usually include several distinctsub-systems: (1) a single-frequency excitation source, such as a laser;(2) an optical probe or region where the light interacts with theanalyte or unknown chemical of interest; (3) a dichroic mirror oroptical filter that blocks the light from the excitation source, lettingonly the Stokes or anti-Stokes scattered light pass; and (4) a spectrumanalyzer or spectrometer that measures the intensity of the Ramanshifted light as a function of frequency or wavelength.

The spectrum analyzer typically includes a dispersive element, such as agrating or prism, that disperses the Raman scattered light for detectionby a detector array. The measured Raman spectrum typically containsseveral peaks, the frequency and intensity of which serve as a unique‘optical fingerprint’ of the chemical being identified. By comparingthis spectrum to a database of known Raman spectra, the composition ofsingle chemicals or mixtures of chemicals in the gas, liquid, or solidphase can be determined with high precision.

In biomedical sensing, Raman spectroscopy is also a promising approachfor non-invasively detecting critical physiological and biochemicalparameters, such as blood glucose, lactate, blood oxygen saturationlevels, etc., owing to its superior chemical selectivity andavailability of near-infrared light sources with sufficient penetrationdepth into biological tissues. Wearable non-invasive blood glucosemonitoring promises great relief to diabetes patients for glucosecontrol but remains an outstanding challenge despite the development ofcommercial glucose meters in the past few decades.

Conventional Raman spectroscopy systems are usually benchtop laboratoryequipment with large size and high cost. Portable Raman spectrometershave slowly begun to enter the market in the last five years, thoughtheir size reduction has relied primarily upon direct miniaturization ofdiscretely-assembled free-space optical components that need to bemanually aligned, such as mirrors, beam-splitters, free-space lenses,and free-space grating spectrometers. These discrete optical componentstypically do not withstand physical shock or vibrations withoutrequiring realignment or recalibration. In addition, these gratingspectrometers suffer from poor sensitivity (which is related to thesignal-to-noise ratio), have limited spectral resolution (typically tono more than 1024 channels), and are relatively large and heavy.

SUMMARY

Embodiments of the present technology generally relate to Fouriertransform Raman spectrometers. In one example, a Fourier transform Ramanspectrometer system includes a light source to emit a probe beam and aprobe waveguide, in optical communication with the light source, toreceive the probe beam and cause at least a portion of the probe beam tointeract with a sample. The interaction between the probe beam and thesample generates a Raman signal. The system also includes a filter inoptical communication with the probe waveguide and configured totransmit the Raman signal and block the probe beam and a Fouriertransform spectrometer in optical communication with the filter. TheFourier transform spectrometer includes a beam splitter to split theprobe beam into a first portion and a second portion, a firstinterference arm in optical communication with the beam splitter, toreceive the first portion of the probe beam, and a second interferencearm in optical communication with the beam splitter to receive thesecond portion of the probe beam. The first interference arm includes afirst optical switch switchable between a first state and a secondstate, a first reference waveguide having a first optical path length L₁to receive the first portion of the probe beam when the first opticalswitch is in the first state, and a first variable waveguide having asecond optical path length L₂, different than the first optical pathlength L₁, to receive the first portion of the probe beam when the firstoptical switch is in the second state. The system also includes adetector, in optical communication with the first interference arm andthe second interference arm, to detect interference of the first portionof the incident light from the first interference arm and the secondportion of the incident light from the second interference arm.

Another embodiment is a Raman spectroscopy system that includes a laser,a first waveguide in optical communication with the laser, a lens inoptical communication with the first waveguide, at least one secondwaveguide in optical communication with the lens, to guide the Ramansignal, a spectrometer in optical communication with the at least onesecond waveguide, and at least one photodetector in opticalcommunication with the spectrometer. These components are on or in asubstrate. In operation, the laser emits a probe beam, which the firstwaveguide guides to the lens. The lens directs the probe beam to asample and collects a Raman signal that is scattered and/or reflectedfrom the sample in response to the probe beam. The second waveguideguides the Raman signal to the spectrometer, which separates the Ramansignal into spectral bins. And the photodetector detects an output ofthe spectrometer.

It should be appreciated that all combinations of the foregoing conceptsand additional concepts discussed in greater detail below (provided suchconcepts are not mutually inconsistent) are contemplated as being partof the inventive subject matter disclosed herein. In particular, allcombinations of claimed subject matter appearing at the end of thisdisclosure are contemplated as being part of the inventive subjectmatter disclosed herein. It should also be appreciated that terminologyexplicitly employed herein that also may appear in any disclosureincorporated by reference should be accorded a meaning most consistentwith the particular concepts disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the drawings primarily are forillustrative purposes and are not intended to limit the scope of theinventive subject matter described herein. The drawings are notnecessarily to scale; in some instances, various aspects of theinventive subject matter disclosed herein may be shown exaggerated orenlarged in the drawings to facilitate an understanding of differentfeatures. In the drawings, like reference characters generally refer tolike features (e.g., functionally similar and/or structurally similarelements).

FIG. 1 shows an energy diagram illustrating changes in frequency ofRaman scattered light that corresponds to the molecular energy levels.

FIG. 2A shows a longitudinal cross section of a miniaturizedFourier-transform Raman spectrometer and chemical analyzer.

FIG. 2B shows a profile view of the miniaturized Fourier-transform Ramanspectrometer and chemical analyzer of FIG. 2A.

FIG. 3 shows a schematic of a miniaturized on-chip FT-Raman spectrometerwith four optical components integrated on a single chip. The dottedlines denote (1a, b) a single-mode, fiber-coupled laser source andoptical isolator edge-coupled to an on-chip waveguide that connects to(2) a spiral-shaped optical probe region, (3) waveguide-integrated Braggreflector (filter), and (4) FT-spectrometer. The waveguide terminates atthe side of the chip and the light is collected by one or two (5)large-area, single-element photodetectors epoxy-bonded to the edge ofthe chip.

FIG. 4 shows a schematic of a miniaturized on-chip FT-Raman spectrometerwith five distinct optical components completely integrated on a singlechip.

FIG. 5A shows a side view of a 3D waveguide-coupled probing/detectingapproach where probe beam are coupled out of and into waveguides via apair of reflective facets and a micro-lens integrated on the waveguidesubstrate to form a prescribed illumination pattern on the analyte.

FIG. 5B shows a top view of a 3D waveguide-coupled FTIR Ramanspectrometer of FIG. 5A.

FIG. 6 shows a side view of a 3D waveguide-coupled probing/detectingapproach where probe beams are coupled out of and into waveguides via apair of curved reflective facets to form a prescribed illuminationpattern on the analyte.

FIG. 7 shows a top view of a 3D waveguide-coupled FTIR Ramanspectrometer with multiple receiving waveguides that couple the Ramansignal from the lens to the spectrometer chip.

DETAILED DESCRIPTION

One method of enhancing the low signal typically generated by Ramanscattering is by using a Fourier-transform spectrometer. AFourier-transform spectrometer is an interferometer that measures anoptical spectrum with a signal-to-noise ratio (SNR) that is √N/2 timeshigher than that of a dispersive grating spectrometer (for a givensampling bandwidth), where N is the number of wavelengths to bemeasured. Current Fourier-transform Raman (FT-Raman) spectrometers arelarge, bulky, benchtop instruments since Fourier-Transform spectrometerstypically include interferometer arms that move back and forth.

Up to now, no complete Raman spectrometer system or Fourier-transformRaman spectrometer system has integrated on-chip photonic sensingelements, filters, and/or spectrometers. Monolithic integration ofoptical components, as described below, results in devices with vastlysmaller form-factors, superior robustness (no moving parts), highspectral resolution, and enhanced sensitivity to chemical species.

Unlike state-of-the-art portable Raman spectrometers with discretefree-space optical components, the spectrometers disclosed hereinexploit monolithic photonic integration with all or most opticalcomponents integrated on one or more planar substrates. Photonicintegration enables dense integration of components without the need formanual alignment or individual component assembly. Photonic integrationalso produces superior mechanical stability and resistance to shock orvibration, which is beneficial to high-performance portable and wearablesensors. Significant performance advantages of photonic integrationinclude (1) a large (e.g., factor of ˜10⁴) improvement in Ramanscattering efficiency resulting from on-chip interaction of the tightlylocalized optical mode and the gas, liquid, or solid analyte, and (2) alarge enhancement in spectral resolution and sensitivity resulting fromthe integration of an on-chip FT-spectrometer. Robust wearablebio-photonic sensors for continuous and non-invasive monitoring ofphysiological and biomedical parameters are also disclosed.

A miniaturized FT-Raman spectrometer can include the followingcomponents: (1) a single-frequency light source; (2) a probe that allowssingle-frequency light (a probe beam) from the single-frequency lightsource to interact with a target sample of interest; (3) one or moreoptical filters to allow a certain band of light to pass (e.g., a filterthat removes the single-frequency light while passing the Raman-shiftedlight); (4) a miniaturized spectrometer that separates received light indifferent spectral bins (e.g., power of light at each Raman-shiftedfrequency); and (5) one or more detectors that measure the outputoptical powers in the spectral bins.

The light source may be a laser that emits light in the visible orinfrared band. The above-mentioned components may be integrated on asingle substrate or multiple substrates. In addition to theabove-mentioned components, the device can further include analog anddigital electronics to power the laser, control the on-chip opticalcircuit, and measure the signal(s) emitted by the photodetector(s) inresponse to incident light. The entire unit can be packaged in aportable form that can be held by hand or carried in one's pocket; wornin a wearable form that can be applied on fingers, wrist, forehead,etc.; or packaged as a ring, a wristband, or an adhesive patch.

The FT-Raman spectrometer system combines a number of on-chip andfiber-/waveguide-connected optical components as described below. First,each component of the on-chip FT-Raman system is described in depth.Lastly, several unique, exemplary embodiments are disclosed.

The laser source that generates the excitation light can be anywavelength in the visible or near-infrared, including, but not limitedto, 457 nm, 473 nm, 488 nm, 514 nm, 532 nm, 633 nm, 660 nm, 785 nm, 830nm, 980 nm, or 1064 nm. Since the Raman scattering intensity is usuallyproportional to 1/λ⁴, with λ being the wavelength of the excitationlight, lower wavelengths are typically preferred. However, using alonger-wavelength laser source, such as a 1064 nm near-infrared Nd:YAGlaser, can suppress fluorescence, thereby decreasing background noiseand enhancing the sensitivity.

Integration of the laser source can be accomplished in at least threeways. The first is directly bonding a waveguide-integrated III/V laseronto the separate FT-Raman chip with the rest of the optical componentsto deliver light directly to an on-chip probe. The second involvesconnecting a separate fiber-coupled laser module to the on-chip probevia tapered edge-coupling or end-fire coupling, or directly connectingthe fiber-coupled laser module to a lensed fiber probe. Additionally,depending on the amount of reflection from subsequent opticalcomponents, a fiber-integrated optical isolator may be used between thelaser and the spectrometer chip to prevent light from reflecting backinto the laser. The third involves direct fabrication of the entireFT-Raman spectrometer, including the laser (such as a DFB laser), on aIII/V material substrate, such as Indium Phosphide (InP).

The optical probe of the miniaturized FT-Raman spectrometer can beconfigured at least in the following ways: (1) a lensed-tip, flat-face,beveled-tip, or ball-lens-tip fiber; (2) an exposed on-chip single-modewaveguide; or (3) an on-chip waveguide including out-of-plane couplingstructures. For the fiber probe, the laser light is first coupled to afiber that passes through an optical circulator or a 1×2 beam combinerand then to the fiber tip. This fiber can then focus the light at thetip, resulting in high optical energy densities that produce Ramanscattering efficiencies comparable to confocal Raman microscopy. Inaddition, the use of fiber probes allows Raman sensing to occur inconfined spaces, making this device useful for medical and surgicaldiagnostics.

In the second probe configuration, light from the excitation source isdirectly coupled to an on-chip single-mode waveguide. This waveguidecontinues to a region where the top-cladding is exposed or where thewaveguide mode profile experiences a change, e.g., due to a change ofrefractive index contrast between the waveguide core and claddingregions or change of waveguide geometries. In this region, light in theevanescent tail of the waveguide mode interacts with nearby gases,liquids, or solids. The Raman signal from this type of probeconfiguration is typically enhanced by four orders of magnitude due tothe dense confinement of light in the single-mode waveguide, the largecollection efficiencies, and the ability to arbitrarily increase theinteraction length by increasing the length of the waveguide. An on-chipwaveguide probe can be configured as a straight waveguide, a paperclipstructure, or a spiral waveguide, which exhibits a long optical pathlength per area with few if any sharp waveguide bends, as shown in FIG.3 (described below).

In the third probe configuration, laser light originally propagating ina probing waveguide is coupled out-of-plane and received by a receivingwaveguide after interacting with the analyte, which allows 3Dwaveguide-coupled sensing of a substance via a planar structure. Such anoptical sensing structure includes a substrate, a light source and lightsource coupler, a probing waveguide, a probing waveguide coupler, areceiving waveguide coupler, a receiving waveguide, a spectrometer chip,and one or more photodetectors.

Basic operation is as follows: light emitted from a laser is firstcoupled into the probing waveguide by the light source coupler viaeither the edge or surface and then propagates inside the waveguide. Theprobing waveguide coupler subsequently redirects the light out of thewaveguide towards the region of interests for sensing. After interactingwith the analyte, scattered or reflected light is collected by thereceiving waveguide coupler and coupled into the receiving waveguide inwhich it propagates to the subsequent elements (e.g., filters,spectrometer chip, photodetectors, etc.). Exemplary probing andreceiving waveguide couplers include reflective facets, curved facetcouplers, micro-lenses, gratings, holography, etc., or combinations ofsuch elements.

The third system includes an on-chip waveguide integrated filter. Theon-chip waveguides and filter can be made on a variety of materialplatforms, such as silicon nitride strip waveguide cores surrounded bysilicon dioxide cladding, a silicon strip waveguide core surrounded bysilicon dioxide cladding, a germanium strip waveguide core surroundedsilicon, or any III-V semiconductor material forming the strip waveguidecore and cladding including, but not limited to InP, InAsP, InGaAsP,etc. This filter reflects light at the laser excitation wavelength,while allowing up to 100% of the Raman scattered light to pass. Inaddition to filtering laser light out of the spectrum, the filter mayreflect the laser light back to the probe, where it may scatteradditional Raman light.

This filter can be realized by several means, such as a periodic Braggreflector. A Bragg reflector includes a waveguide with a width orcladding that is modulated with a period such that light at theexcitation wavelength is reflected with high efficiency. Additionalmeans include, but are not limited to, planar concave gratings,arrayed-waveguide gratings, echelle gratings, angled multi-modeinterferometers, ring cavity filters, Mach-Zehnder interferometerfilters, and Vernier effect filters.

Another method of filtering out the light at the excitation wavelengthis by narrowing the waveguide width such that the mode's cutofffrequency lies below the frequency of the excitation source. Thewavelength of the excitation is usually shorter than the wavelength ofthe Stokes line. This technique works for Raman spectroscopy measuringanti-Stokes shifted light. Such waveguide-integrated filters arelithographically fabricated in parallel with the other on-chip opticalcomponents.

The Raman scattered light then travels via the optical waveguide to anon-chip FT-spectrometer utilizing the same material platform asdescribed before. The FT-spectrometer can be monolithically defined on aplanar substrate or chip, with no moving parts. The schematic is shownin FIGS. 3 and 4, described below. The waveguide after the filter issplit into two interferometer arms by an on-chip beam-splitter or 1×2multi-mode interferometer (MMI). Each interferometer arm has a discretenumber of repeated units that each includes an optical switch thatguides light into one of two waveguides with different path lengths.

Each optical switch can take the form of a Mach-Zehnder interferometerwith phase modulators that cause the light to transmit through only oneof the two beam-combiner, or 2×2 MMI, outputs. The last component of theinterferometer is a 2×2 MIMI that combines the light from both arms ofthe interferometer. Each output of the 2×2 MMI provides informationabout the magnitude and phase of the interferogram. Each optical switchin the FT-spectrometer has two states, guiding light to the ‘top’ or the‘bottom’ path. Thus, for a FT-spectrometer as shown in FIGS. 3 and 4with six optical switches, there are 2⁶=64 unique interferometer opticalpath length differences, corresponding to 64 measurable spectral datapoints. This type of scaling means that a 40% increase in chip-size(going from six switches to ten switches) results in a spectralresolution of 2¹⁰=1024 data points.

More information about the Fourier transform spectrometer can be foundin U.S. patent application Ser. No. 15/429,321, entitled “APPARATUS,SYSTEMS, AND METHODS FOR ON-CHIP SPECTROSCOPY USING OPTICAL SWITCHES,”which is hereby incorporated herein by reference in its entirety.

Traditional linear detector arrays also have electrical wiring andcontrol of all 1024+ photodetectors. Conversely, integration of thisparticular type of FT-spectrometer means the same resolution can beaccomplished for a small fraction of the chip-space and electricalrouting of twenty 2-terminal phase modulators (2 phase modulatorsoptical switch) and one 2-terminal photodetector, rather than electricalrouting of 1024 2-terminal photodetectors.

The output of the on-chip FT-spectrometer is routed to one or twosingle-element photodetectors. These may be on-chip waveguide integrateddetectors, such as germanium on silicon photodetectors, or off-chipsingle element detectors. These may include silicon, indium galliumarsenide (InGaAs), or germanium detectors, or any other detector capableof detecting light in this band. Light from the waveguide can be coupledto a fiber or waveguide that is then connected to the photodetector. Asecond possible embodiment includes on-chip detectors that are hybridbonded or flip-chip bonded to the surface of the chip with theFT-spectrometer. A third embodiment of this component is directepoxy-bonding of a large, single element detector to the edge of thespectrometer chip. This last embodiment is low-cost, easy to scale formanufacturing (precise optical alignment is not necessary), and morerobust than fiber-coupling.

FIGS. 2A and 2B show a thin, pen-sized spectrometer 200 that can beeasily carried by hand or in a person's pocket. Pointing the device 200at a substance of unknown chemical composition and pressing a button 217on the side begins the process of acquiring a Raman spectrum. Thespectral information is then sent to a nearby cell-phone, computer, orserver for spectral analysis. The spectrum is cross-referenced against aknown database of Raman spectra, and information about the type ofchemical, the concentration, and the match quality (such as hit-qualityindex, or HQI) is presented to the user on the FT-Raman spectrometer oron the user's cell-phone screen. The excitation light delivery can alsobe done via lenses. One advantage of using a lens is that the system maybe free of circulators.

FIG. 2A shows a cross section of the pen-sized spectrometer 200. Itincludes a single-frequency laser 213 that is connected to a 2×1 beamcombiner 208 via a first single-mode fiber connector 214. The otherports of the 2×1 beam combiner 208 are coupled to a lensed or taperedfiber tip 210 via a second single-mode fiber connector 209 and to anedge-, butt-, or grating coupler 206 via a third single-mode fiberconnector 207. The coupler 206 is coupled to an on-chip band-pass orlow-pass filer 205, which in turn is coupled to an on-chipFourier-transform infrared (FTIR) spectrometer 204. The output of theFTIR spectrometer 204 is coupled a broadband photodiode or photodetector201 via another coupler 203 and a fourth single-mode fiber coupler 202.The device 200 may also include an electronic printed circuit board 211with electronic for controlling the laser 213 and FTIR spectrometer 204for reading signals from the photodetector 201 and an antenna forcommunicating with a cell phone or other wireless device. Thesecomponents may be included in a housing 216, along with a power supply215, that includes openings for a button 217 that actuates thespectrometer 200 and a screen 218 for displaying the analysis results.

From FIG. 2, the detailed operation of the device is as follows: first,the user brings the tip of the lensed or tapered fiber 210 within closeproximity to the substance to be analyzed, and then presses an externalbutton 217 to begin the scan and identify the types of chemicalspresent. Pressing the button turns on the single frequency laser sourcein the visible or near-infrared wavelength range 213, which sends light(a probe beam) down the first single-mode fiber 214 and through thefiber beam combiner or optical circulator 208 and to a secondsingle-mode fiber 209. The light exits the fiber 209 through the lensedor tapered fiber tip 210 and interacts with the substance to beidentified (not shown). The lensed or tapered fiber tip 210 collects thescattered Raman light, which travels back down the single-mode fiberconnector 209 to the fiber beam combiner or optical circulator 208,which couples it into another single-mode fiber connector 207.

Here, the light enters an on-chip waveguide (not shown) through eitheran edge-coupler, butt-coupler, or grating coupler 206. On the opticalchip, the light passes through the waveguide integrated band-pass filteror an on-chip low-pass filter 205, which can take the form of a Bragggrating, ring resonator, selectively absorbing element, or narrowwaveguide with specific cutoff frequency. After this, the light travelsto the on-chip FTIR spectrometer 204. After light passes through theinterferometer, it exits in either one or two separate waveguides (notshown). The light in these one or two waveguides exit the photonic chipthrough another edge coupler, butt-coupler, grating coupler, orfree-space lens 203 (or other free space coupling mechanisms). Thislight then passes through one or two single mode fibers 202 and ismeasured by one or two broadband photodetectors 201.

This procedure is performed several different times, for differentconfigurations of the on-chip FTIR. Digital and analog electronics 211measure the signal from the photodetector 201, power the excitationlaser 213, and control the optical switches in the on-chip FTIRspectrometer 204. In addition, the electronics 211 digitally calculatethe spectrum of the Raman shifted light and send the data to a separatedevice (e.g., a cell-phone, computer, or server) that compares thisspectrum to a spectral database. Once the chemical compositions havebeen determined, they are sent back to the pen-shaped FT-Ramanspectrometer 200 and the result is either displayed on a front-paneldisplay 218. Alternatively, the other device (e.g., the cell phone) maydisplay the results directly instead of or in addition to sharing themwith the spectrometer 200.

FIG. 3 shows an example integrated on-chip FT-Raman spectrometer system300 with many components integrated on-chip. In this system, asingle-mode fiber-coupled source laser 310 a is coupled to an opticalisolator 310 b (to prevent reflections at the chip facet from damagingthe laser diode). An output fiber 312 from the isolator 310 b is affixedvia epoxy to the edge of a spectrometer chip 302 after optical alignmentto an on-chip waveguide 304. Light from the laser 310 a (a probe beam)travels through a spiral probe region 320 of the waveguide 304 whereRaman scattered light is generated from gases or liquids (not shown) incontact with the chip 302. This light then passes through an on-chipfilter 330, which transmits the Raman signal and reflects the sourcelight back to the probe region 320, where it can generate more Ramanscattered light. The transmitted Raman signal light is then decomposedinto the spectral components via a Fourier-transform spectrometer 340,which includes optical switches 342 (black boxes), phase modulators 344(white boxes), and waveguides 346 of different path lengths in two arms341 as described above. The output of the on-chip FTIR spectrometer 340is measured using one or two large-area single element detectors 350epoxy-bonded to the edge of the spectrometer chip 302. Not shown in FIG.3 is the electrical wiring of the laser 310 a, detector 350, and on-chipphase modulators 344. This chip 302 can be electrically connected to aseparate board containing the analog and digital electronics for controland signal readout.

FIG. 4 shows a completely integrated on-chip FT-Raman spectrometersystem 400. This particular embodiment contains all five opticalcomponents waveguide integrated on a single planar substrate 402. Thedotted boxes denote a laser source 410 coupled to an on-chip opticalwaveguide 404, routed to a spiral-shaped probe region 420, where lighttravels a relatively large distance interacting with gas and liquidmolecules, then routed to an on-chip optical filter 430 that removes thelaser frequency light and allows the Raman-shifted light to pass to anon-chip FTIR spectrometer 440 with discretely tunable arm lengths, andfinally to one or more detector elements 450 that enable readout of thesignal for each wavelength. Again, the on-chip FTIR spectrometer 440includes phase modulators 444 (white boxes) and waveguides 446 ofdifferent path lengths coupled to optical switches 442 (black boxes).FIG. 4 doesn't show the electrical wiring of the laser 410, detector(s)450, or on-chip phase modulators 444. This chip 402 can be electricallyconnected to a separate board containing the analog and digitalelectronics for control and signal readout.

One advantage of this integrated spectrometer 400 shown in FIG. 4 isthat the high degree of miniaturization means that the system can befree of optical fibers. In addition, the system can be especiallyresistant to physical shock or vibration. Significant decreases inoptical coupling losses are possible by integrating many or all of thecomponents on a single chip. In addition, the spiral waveguide probestructure 420 results in a ˜10⁴ enhancement in the device's ultimatesensitivity. This device 400 is ideal for monitoring the concentrationof various organic chemicals or pollutants in the gas or liquid phase.Applications include air-quality monitoring, leak detection in oil andgas systems, early-warning-systems for contamination of freshwaterbodies of water, and monitoring of different research or industryprocesses involving unknown chemicals or biological species.

Examples of the above-mentioned 3D waveguide-coupled sensing approachare also disclosed. In one example spectrometer 500, shown in FIGS. 5Aand 5B, a laser 510 launches a probe beam into a probing waveguide 520integrated in or on a substrate 502. The probing waveguide 520 guidesthe probe beam to a reflective waveguide facet 522 a formed at the endof the probing waveguide 522 b and a micro-lens 524 formed on thebackside of the waveguide substrate 502. The probe beam is firstredirected by the facet coupler 522 a to propagate out-of-plane andilluminates an analyte 501 via the micro-lens 524, which generates aprescribed illumination pattern (e.g., collimated, focused, orstructured etc.). For example, the waveguide micro-optical couplers 522a and 522 b can act as confocal imagers and spatially filter out noisefrom background and ambient light and suppress inter-channel cross-talk.

The scattered or reflected light, including the Raman signal, iscollected by the micro-lens 524 and a receiving waveguide coupler 522 b,which converts the probe beam into a waveguide mode in a receivingwaveguide 526. The micro-lens 524 may also be formed on top of thewaveguides 520 and 526 by properly configuring the waveguide couplers522 a and 522 b. On-chip filters and spectrometer 540 block anycollected probe light and separate the collected Raman signal intodifferent spectral bins for detection by one or more photodetectors 550.Electronics (not shown), which are powered by a battery 515, control thelaser 510 and spectrometer 540 and process the signals from thephotodetector 550.

FIG. 6 shows an integrated spectrometer 600 with curved waveguide facets622 a and 622 b for probing an analyte 601 and collecting a Raman signalfrom the analyte. The integrated spectrometer 600 has a laser 610 thatlaunches a probe beam into a probing waveguide 620 that ends in a firstcurved waveguide facet 622 a that reflects and focuses the probe beamonto the analyte 601. A second curved waveguide facet 622 b collects thescattered or reflected light from the analyte 601 and couples it into areceiving waveguide 626.

The facets 622 a and 622 b can be curved or shaped to focus or collimatelight and may have the same or different curvatures. For instance, thefirst facet 622 a may be shaped to focus the probe beam on or just belowthe surface of the analyte 601. And the second facet 622 b may be shapedto couple the scattered or reflected light into the receiving waveguide626 based on the receiving waveguide's numerical aperture. The facets622 and 622 b can also be configured to re-direct the light to eitherside of the waveguide plane.

As explained above, the receiving waveguide 626 couples the collectedlight into on-chip filters and a spectrometer 640, which block anycollected probe light and separate the collected Raman signal intodifferent spectral bins for detection by one or more photodetectors 650.Electronics (not shown) control the laser 610 and spectrometer 640 andprocess the signals from the photodetector 650.

The input laser light may be distributed into multiple probing channelsto probe different regions of the analyte, which allows differentiatingRaman scattered light, directly reflected light, and background noise bypost-processing the data collected in each channel. Multiple receivingwaveguides and waveguide couplers may be used to enhance the collectionefficiency of Raman scattered light. For example, FIG. 7 shows amodified version 700 of the spectrometer 500 in FIGS. 5A and 5B withmany receiving facets 722 and receiving waveguides 726 for collectinglight from different regions or angles. The receiving facets 722 arearranged in a ring-shaped array around the probe facet 522 a and couplelight to respective receiving waveguides 726, which are coupled to thespectrometer and filters 540. They further improve the collection ofscattered Raman light while spatially filtering out secularly reflectedprobing light. Other arrangements are also possible, for example,including asymmetric arrays of waveguides. Similarly, other types offacets are also possible—the waveguides 722 in FIG. 7 receive light viaa micro-lens, but could be curved instead (e.g., as in FIG. 6). They canalso be used with multiple illuminating facets, or there could bemultiple illuminating facets and only one collecting facet.

The 3D waveguide-coupled probing/detecting approach is particularlyattractive to wearable or conformal sensors as it effectively bringsconfocal imaging onto the surface of a substance while utilizing planarwaveguide structures and integrated photonic components. Theout-of-plane coupled waveguide structure can be combined with theevanescently-coupled waveguide structures to form a 2D/3D hybrid sensingscheme.

CONCLUSION

While various inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the inventive embodiments describedherein. More generally, those skilled in the art will readily appreciatethat all parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the inventive teachingsis/are used. Those skilled in the art will recognize or be able toascertain, using no more than routine experimentation, many equivalentsto the specific inventive embodiments described herein. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, inventive embodiments may be practicedotherwise than as specifically described and claimed. Inventiveembodiments of the present disclosure are directed to each individualfeature, system, article, material, kit, and/or method described herein.In addition, any combination of two or more such features, systems,articles, materials, kits, and/or methods, if such features, systems,articles, materials, kits, and/or methods are not mutually inconsistent,is included within the inventive scope of the present disclosure.

Also, various inventive concepts may be embodied as one or more methods,of which an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

The invention claimed is:
 1. A system, comprising: a light source to emit a probe beam; a probe waveguide, in optical communication with the light source, to receive the probe beam and cause at least a portion of the probe beam to interact with a sample, the interaction between the probe beam and the sample generating a Raman signal; a filter, in optical communication with the sample, to transmit the Raman signal and block the probe beam; a Fourier transform spectrometer in optical communication with the filter, the Fourier transform spectrometer comprising: a beam splitter to split the Raman signal into a first portion and a second portion; a first interference arm, in optical communication with the beam splitter, to receive the first portion of the Raman Signal, the first interference arm comprising: a first optical switch switchable between a first state and a second state; a first reference waveguide having a first optical path length L₁ to receive the first portion of the Raman signal when the first optical switch is in the first state; and a first variable waveguide having a second optical path length L₂, different than the first optical path length L₁, to receive the first portion of the Raman signal when the first optical switch is in the second state; and a second interference arm, in optical communication with the beam splitter, to receive the second portion of the Raman Signal; and at least one detector, in optical communication with the first interference arm and the second interference arm, to detect interference of the first portion of the Raman signal from the first interference arm and the second portion of the Raman signal from the second interference arm.
 2. The system of claim 1, wherein the probe waveguide comprises a spiral-shaped waveguide section.
 3. The system of claim 1, further comprising: a lens, in optical communication with the probe waveguide, to focus the probe beam toward the sample.
 4. The system of claim 3, wherein the lens is configured to collect the Raman signal from the sample, and further comprising: at least one other waveguide, in optical communication with the lens, the couple the Raman signal from the lens to the filter.
 5. The system of claim 4, wherein the at least one other waveguide comprises: an array of waveguides in optical communication with the sample, each waveguide in the array of waveguides being configured to receive a corresponding portion of the Raman signal from the sample.
 6. The system of claim 1, wherein the probe waveguide comprises a first curved facet to focus the probe beam toward the sample.
 7. The sample of claim 6, further comprising: another waveguide, in optical communication with the sample and the filter, the other waveguide having a second curved facet to collect the Raman signal from the sample.
 8. The system of claim 1, wherein the filter comprises a Bragg reflector.
 9. The system of claim 1, further comprising: a beam combiner in optical communication with the first interference arm and the second interference arm, the beam combiner comprising: a first input, in optical communication with the first interference arm, to receive the first portion of the probe beam; a second input, in optical communication with the second interference arm, to receive the second portion of the probe beam; a first output; and a second output, wherein the at least one detector comprises: a first detector in optical communication with the first output; and a second detector in optical communication with the second output.
 10. The system of claim 1, further comprising: a substrate, wherein the at least one probe waveguide, the filter, the Fourier transform spectrometer, and the at least one detector are fabricated on or in the substrate.
 11. A method of Raman spectroscopy, the method comprising: emitting a probe beam from a laser; guiding the probe beam from the laser to a sample via a first waveguide integrated in or on a substrate; coupling the probe beam out of the first waveguide to a sample, the probe beam causing the sample to generate a Raman signal; coupling the Raman signal into a second waveguide integrated in or on the substrate; splitting the Raman signal into a first portion and a second portion; guiding the first portion through a first interference arm integrated in or on the substrate, the first interference arm comprising: an optical switch switchable between a first state and a second state; a reference waveguide having a first optical path length L1 to receive the first portion when the first optical switch is in the first state; and a variable waveguide having a second optical path length L2, different than the first optical path length L1, to receive the first portion when the first optical switch is in the second state; and guiding the second portion through a second interference arm integrated in or on the substrate; and interfering the first portion and the second portion at a detector coupled to the first interference arm and the second interference arm.
 12. The method of claim 11, further comprising: switching the optical switch between the first state and the second state.
 13. The method of claim 11, wherein coupling the probe beam out of the first waveguide to the sample comprises focusing the probe beam toward the sample with a lens integrated in or on the substrate.
 14. The method of claim 13, wherein coupling the Raman signal into the second waveguide comprises collecting the Raman signal from the sample with the lens.
 15. The method of claim 11, wherein coupling the Raman signal into the second waveguide further comprises coupling respective portions of the Raman signal into respective waveguides in an array of waveguides integrated in or on the substrate.
 16. The method of claim 11, wherein coupling the probe beam out of the first waveguide to the sample comprises focusing the probe beam with a curved facet at an end of the first waveguide.
 17. The method of claim 11, wherein coupling the Raman signal into the second waveguide comprises reflecting the Raman signal off a second curved facet into the second waveguide.
 18. A Raman spectroscopy system comprising: a substrate; a laser, integrated on the substrate, to emit a probe beam; a first waveguide, integrated on or in the substrate in optical communication with the laser, to guide the probe beam; a lens, integrated on or in the substrate in optical communication with the first waveguide, to direct the probe beam to a sample and to collect a Raman signal from the sample in response to the probe beam; at least one second waveguide, integrated on or in the substrate in optical communication with the lens, to guide the Raman signal; a spectrometer, integrated on or in the substrate in optical communication with the at least one second waveguide, to separate the Raman signal into spectral bins; and at least one photodetector, integrated on or in the substrate in optical communication with the spectrometer, to detect an output of the spectrometer, wherein the spectrometer comprises: an optical switch switchable between a first state and a second state; a reference waveguide having a first optical path length L1 to receive a first portion of the Raman signal when the first optical switch is in the first state; and a variable waveguide having a second optical path length L2, different than the first optical path length L1, to receive the first portion when the first optical switch is in the second state.
 19. The Raman spectroscopy system of claim 18, wherein the at least one second waveguide comprises an array of second waveguides, each of which is configured to guide a corresponding portion of the Raman signal. 